"So in summary... Google has pulled the plug on support on a protocol they've helped popularize, after years of promising interoperability, for reasons that are dubious at best, and in a way that leaves people who don't jump to the new Hangouts app unable to talk to their contacts without any feedback that their IMs aren't getting through... And they've done that with no warning to anyone. I imagine there's a bunch of people out there wondering where some of their buddies have gone, or why their messages aren't getting responses, because this isn't documented anywhere." Google really messed this up. Such a dick move.

I work full time with Microsoft Lync, while this won't make a difference in most of the deployments I work with... there are some for which this is going to cause real headaches.
It seems a really bizarre choice on Google's part

Also the lack of user-selectable presence information (status) is a massive point of pain.